For example, a valve body that constitutes a spool valve has been manufactured by pouring a molten metal (principally a molten metal of an aluminum alloy) into a cavity of a casting die device, and allowing the molten metal to harden. Stated otherwise, the valve body is obtained as a cast product.
In the case of this type of valve body, a valve hole (inner bore) is formed for slidable insertion therein of a spool that makes up a valve member. At least one end of the valve hole is opened at a predetermined location in the valve body so as to allow the spool to be inserted therein.
The valve hole is formed, for example, by a core pin. More specifically, the core pin is inserted beforehand into the interior of a cavity, and in this state, the molten metal is poured into the cavity. Then, after the molten metal has hardened and a cast product is obtained, the core pin is removed or separated away from the cast product, whereby a hollow portion is formed having a shape corresponding to the shape of the core pin. The hollow portion serves as the valve hole.
In this case, on the casting surface of the valve hole, typically casting defects such as blowholes or flow lines are formed therein. Therefore, with respect to an inner wall of the valve hole, a location up to a depth of about 0.5 mm to 1 mm is removed by a grinding process, and an operation to expose the inner part of the inner wall is carried out extensively. More specifically, in the spool valve as a product to be distributed, the surface of the inner wall of the valve hole is a machined surface that is exposed by grinding.
However, on such a machined surface, cases may occur in which casting defects such as blowholes or the like that exist in the vicinity of the machined surface (in inner layers of the valve hole) become exposed. Consequently, for eliminating casting defects in the machined surface, there is a need to reduce the occurrence of such casting defects as much as possible in the inner layers of the valve hole.
In Japanese Laid-Open Patent Publication No. 2000-238041, it is disclosed to immerse a die to which ultrasonic vibrations are applied into a molten metal. According to the disclosure of Japanese Laid-Open Patent Publication No. 2000-238041, in this condition, when the die is pulled out from the molten metal, a state is maintained in which the molten metal adheres to the die. Further, according to Japanese Laid-Open Patent Publication No. 2000-238041, by continuing to apply the ultrasonic vibrations until a certain degree of hardening has taken place following die matching (die closure), it is disclosed that the occurrence of casting defects such as blowholes, flow lines or the like can be reduced.
However, even if vibrations are applied to the die as disclosed in Japanese Laid-Open Patent Publication No. 2000-238041, such vibrations often are not transmitted sufficiently to the molten metal. More specifically, merely by imparting vibrations to the mold, it is not easy for casting defects in the inner wall and inner layers of the inner bore to be reduced.